Field of the Invention
The invention relates to a semiconductor device with ohmic contact-connection and also to a method for the ohmic contact-connection of a semiconductor device.
The invention relates, in particular, to a semiconductor device which is composed of silicon carbide (SiC) at least in a semiconductor region which is contact-connected.
Silicon carbide in monocrystalline form is a semiconductor material having outstanding physical properties. On account of its high breakdown field strength, SiC is a semiconductor material of interest, inter alia, particularly for power electronics, even for applications in the kV range. Owing to the large energy gap, which also enables emission or detection of short-wave light in the blue or ultraviolet spectral region, SiC also constitutes a promising semiconductor material for optoelectronics.
Since the commercial availability of wafers made of monocrystalline silicon carbide, in particular of the 6H and 4H polytypes, and also the technological understanding of SiC have increased, SiC components are now also receiving more and more attention. Thus, a description has already been given e.g. of a Schottky diode, a pn diode, various transistors such as a MOSFET (Metal Oxide Semiconductor Field Effect Transistor), a MESFET (Metal Semiconductor Field Effect Transistor) or a JFET (Junction Field Effect Transistor), but also an LED (Light Emitting Diode), a laser diode or a photodetector, in each case based on silicon carbide.
In each case at least one stable ohmic contact on a semiconductor region made of SiC is required for the functioning of these components. This at least one ohmic contact is situated e.g. on a back side of the wafer. In this case, the lowest possible contact resistance in conjunction with the smallest possible contact area is sought in order to avoid undesirable losses at the semiconductor-metal junction.
The review articles xe2x80x9cOhmic contacts to SiCxe2x80x9d by G. L. Harris et al. from xe2x80x9cProperties of Silicon Carbidexe2x80x9d ed. by G. L. Harris INSPEC, 1995, pages 231-234 and xe2x80x9cA critical review of ohmic and rectifying contacts for silicon carbidexe2x80x9d by L. M. Porter and R. F. Davis, Materials Science and Engineering, B34, 1995, pages 83-105 contain summaries of contact-connection methods for silicon carbide of different polytypes and conduction types. Accordingly, nickel has been used the most often hitherto as a contact material for an ohmic contact on n-conducting SiC. After the nickel material has been applied, a heat-treatment process for forming the ohmic contact is usually performed at a process temperature of above 900xc2x0 C. The lowest documented contact resistance for n-conducting SiC is 1.10xe2x88x926 xcexa9cm2. In this embodiment, the nickel contact is heat-treated for five minutes at 1000xc2x0 C. However, the substrate which has undergone ohmic contact-connection is composed of n-conducting 6Hxe2x80x94SiC having a high dopant concentration of 4.5xc2x71020 cmxe2x88x923, which is not very practicable. The nickel contact is situated on the (000{overscore (1)}) face, i.e. on the carbon face, of the 6Hxe2x80x94SiC substrate.
U.S. Pat. No. 3,510,733 describes an ohmic contact between a lead wire and a semiconductor region made of n-conducting silicon carbide. The lead wire is composed either of pure chromium or of 20% chromium and 80% nickel or of 15% chromium, 60% nickel and 25% iron or of stainless steel with a material portion of from 11 to 20% chromium, up to 12% nickel, up to 2% magnesium, up to 1% silicon, up to 0.3% carbon and with a main proportion of iron. The main requirement placed on the material of the lead wire and of the ohmic contact is sufficient ductility and resistance to oxidation even at a high temperature. Connecting the cross-sectional area of the lead wire to the semiconductor region results in a punctiform contact delimited, in particular, by the wire geometry, such as e.g. diameter. The lead wires used have a diameter of 0.0508 mm and 0.127 mm. The ohmic connection between the lead wire and the semiconductor region made of silicon carbide is achieved by heating above the melting point of the materials used. These temperatures lie between 1500 and 1900xc2x0 C. However, the process conditions during the production of a semiconductor device made of silicon carbide often limit a maximum permissible process temperature for achieving an ohmic contact to a distinctly lower value.
It is accordingly an object of the invention to provide a semiconductor device with ohmic contact-connection and method for the ohmic contact-connection of a semiconductor device that overcomes the above-mentioned disadvantages of the prior art methods and devices of this general type, in which the favorable properties of silicon carbide are utilized to practical advantage and the contact-connection of n-conducting SiC is improved in comparison with the prior art.
It is also an object of this invention, to provide a method for the ohmic contact-connection of a semiconductor device that overcomes the above-mentioned disadvantages of the prior art methods and devices of this general type. In particular, the contact on the semiconductor region is intended to be heated during the heat-treatment process to a lower process temperature than in the prior art, but at least a comparable contact resistance to that in the prior art is nonetheless intended to be achievable.
With the foregoing and other objects in view there is provided, in accordance with the invention a semiconductor device with ohmic contact-connection comprising at least
a semiconductor region made of n-conducting silicon carbide,
adjoining the semiconductor region a largely homogeneous ohmic contact layer made of a material having a first component able to form a silicide at a process temperature of at most 1000xc2x0 C., and a second component able to form a carbide at a process temperature of at most 1000xc2x0 C.,
and a junction region extending into the semiconductor region and into the ohmic contact layer, containing a silicide formed from the first component and the silicon of the silicon carbide and a carbide formed from the second component and the carbon of the silicon carbide.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for the contact-connection of a semiconductor device which comprises
applying a largely homogeneous ohmic contact layer made of a material having a first component able to form a silicide and a second component able to form a carbide to a semiconductor region made of n-conducting silicon carbide, the two material components being applied simultaneously, and
subjecting the structure comprising semiconductor region and ohmic contact layer to a heat-treatment process with heating to a process temperature of at most 1000xc2x0 C.,
whereby a silicide is formed from the first material component and the silicon of the silicon carbide, and a carbide is formed from the second material component and the carbon of the silicon carbide, in a junction region extending into the semiconductor region and into the ohmic contact layer.
The invention is based on finding that it is possible to produce a stable ohmic contact to a semiconductor region made of n-conducting silicon carbide with very low contact resistance from a material comprising two material components, if one material component forms a silicide with the silicon of the silicon carbide and the other material component forms a carbide with the carbon of the silicon carbide. Consequently, a quaternary material system comprising the silicon and the carbon of the silicon carbide and also the two material components is present in the junction region extending both into the semiconductor region and into the ohmic contact layer. Consequently, after severing of the atomic bond between the silicon and the carbon of the silicon carbide at an elevated temperature, there is available for each of the two elements a new bonding partner with which a silicide and a carbide, respectively, can be formed.
This silicide and carbide formation essentially takes place during the heat-treatment process which is carried out after applying the material. However, it is also the case that initial nucleation for the silicide and the carbide can already occur even during the material application depending on the prevailing process conditions (deposition temperature, energy content of material particles produced by sputtering) at the interface with the SiC.
In the case of a pure nickel contact in accordance with the prior art, nickel silicide is formed during the heat-treatment process. In this case, however, carbon atoms then remain in the junction region on account of the stoichiometric ratio between silicon and carbon in the silicon carbide. These atoms then form graphite inclusions with relatively poor conductivity. This results in an unfavorable influence on the ohmic contact behavior. According to the invention, in contrast, this is improved by providing a bonding partner for the formation of a carbide from the carbon atoms.
Since the silicide and carbide formation essentially take place during the heat-treatment process, it is helpful if both the first and the second material components are already situated at the interface with the semiconductor region to a sufficient extent before the beginning of the heat-treatment process. This is advantageously achieved by both material components being applied simultaneously. In this case, the material may be present in the form of a mixture, a batch, an alloy or a compound of these two material components as a practically homogeneous layer on the semiconductor region. The silicide and the carbide can then be formed at any time in the heat-treatment process, in particular also immediately after the start of the heat treatment.
The two material components are advantageously chosen such that the silicide and the carbide are formed during the heat-treatment process at a maximum process temperature of at most 1000xc2x0 C. With this process temperature, it is then typically possible to achieve a contact resistance of the order of magnitude of 1xc2x710xe2x88x927 xcexa9cm2. As a result, the best contact resistance mentioned in the prior art is improved approximately by one order of magnitude.
After the heat-treatment process, a slightly inhomogeneous material composition will be present within the contact layer. This stems from exchange processes between the individual material components in the junction region. Specifically, the silicide and carbide formation in this case result in an exchange of atoms which were originally assigned to the semiconductor region and the ohmic contact layer, which was initially applied in a practically homogeneous manner. After the conclusion of the heat-treatment process, it is thus the case that, depending on the process control chosen, in the junction region the ohmic contact layer deviates from the homogeneous material composition present in the remaining region of the ohmic contact layer. The expression xe2x80x9clargely homogeneous ohmic contact layerxe2x80x9d should therefore then be understood such that this inhomogeneity is concomitantly included.
Moreover, differences in the material composition which are to be attributed to customary contamination in starting substances are likewise regarded as non-critical here.
The ohmic contact-connection according to the invention can advantageously be used for a semiconductor device made of silicon carbide in a wide variety of embodiments. Possible embodiments of an SiC semiconductor device are e.g. a Schottky diode, a pn diode, a MOSFET, a MESFET, a JFET, an LED, a laser diode or a photodetector. Further embodiments are likewise possible.
Particular refinements of the semiconductor device and of the method according to the invention emerge from the respective dependent claims.
In a preferred embodiment, the second material component is present with a proportion by volume of from 2 to 50% in the material. A proportion of from 10 to 30% is particularly preferred.
To form a good ohmic contact, it is advantageous if the n-conducting semiconductor region has a sufficiently high dopant concentration at least at the interface with the ohmic contact layer. The dopant concentration preferably lies between 1017 cmxe2x88x923 and 1020 cmxe2x88x923. A particularly good ohmic contact results if the dopant concentration is at least 1019 cmxe2x88x923. Nitrogen or phosphorus are used as dopant in the n-conducting semiconductor region.
An embodiment in which the two material components form the silicide and the carbide at a process temperature as low as 900xc2x0 C., or even below 900xc2x0 C., is advantageous. As a result, the temperature during the heat treatment can be reduced compared with the prior art, without this being associated with a loss in the contact resistance that can be achieved. A contact resistance of at most 5xc2x710xe2x88x927 xcexa9cm2 can be achieved in a simple manner with a process temperature of approximately 900xc2x0 C. At a process temperature that is reduced further, for example 850xc2x0 C., a contact resistance of the order of magnitude of 1xc2x710xe2x88x926 xcexa9cm2 is still obtained.
In another advantageous refinement, the first material component is composed of nickel (Ni) or cobalt (Co). A variant in which the second material component is iron (Fe), tungsten (W), vanadium (V) or tantalum (Ta) is additionally favorable. These materials are silicide- or carbide-forming materials, so that they constitute particularly suitable contact materials for the semiconductor device. Preferred material combinations are permalloy (NiFe) or a cobalt/iron (CoFe) alloy.
The ohmic contact layer preferably has a thickness in the range from 15 to 200 nm. A material application having such a thickness can be realized without difficulty. At the same time, this contact layer thickness ensures that there is a sufficient quantity of the two material components for the formation of silicide and carbide. A good ohmic characteristic of the contact is obtained in this way. In an advantageous manner, the contact layer is at least thick enough that the further processing steps can be performed on the contact layer. On the other hand, the time and expense required place upper limits on the thickness. A material application with a thickness of 100 nm effected by sputtering lasts about 20 minutes. Moreover, the contact layer is at most so thick that a possible downstream lift-off process step is still possible without special precautions.
The n-conducting semiconductor region that is to be contact-connected can comprise different SiC polytypes. There are embodiment variants in which xcex1-SiC e.g. in the form of 6Hxe2x80x94, 4Hxe2x80x94, or 15R-SiC or xcex2-SiC in the form of 3Cxe2x80x94SiC is used for the n-conducting semiconductor region. However, other polytypes are likewise possible.
An xcex1-Sic single crystal has two mutually opposite crystal faces which are particularly distinguished by the crystal geometry and are usually designated by (0001)xe2x80x94or silicon-face and by (000{overscore (1)})xe2x80x94or carbon-face. There is, then, an embodiment of the semiconductor device in which the ohmic contact layer is arranged on the (0001) face of an n-conducting xcex1-SiC semiconductor region, and also another embodiment in which the ohmic contact layer is situated on the (000{overscore (1)}) face. The contact layer made of a silicide- and a carbide-forming material component, in particular a contact layer made of a nickel/iron alloy, has the particular property that, in contrast to the pure nickel contact layer used inter alia in the prior art, it leads to an equally good contact behavior both on the (0001) face and on the (000{overscore (1)}) face.
Outside the n-conductor semiconductor region, the semiconductor device may also be composed, at least in some regions, of a material other than SiC, for example of silicon (Si) or gallium arsenide (GaAs). A vertical LED or laser diode can also comprise a light-emitting gallium nitride (GaN) layer structure on an n-conducting SiC substrate. On that side of the SiC substrate which is remote from the light-emitting GaN layer structure, an ohmic contact is required via which an electric current can be fed into the light-emitting layer structure.
In a further advantageous embodiment of the semiconductor device, a covering layer is situated on a side of the ohmic contact layer which is remote from the n-conducting SiC semiconductor region. Such a covering layer can also have a protective function. It covers the contact layer and thereby protects the latter against an undesirable influence during a further process stepxe2x80x94optionally taking place after the ohmic contact-connectionxe2x80x94for fabricating the semiconductor device. Thus, e.g. during a subsequent treatment with hydrofluoric acid, the covering layer prevents a direct contact and thus a chemical reaction of the contact layer with the hydrofluoric acid.
The covering layer therefore contains, in particular, a metallic material which, moreover, is preferably chemically inert with respect to an aggressive substance used, for instance, in a subsequent process step. In addition, the metal used is practically stable, chemically and physically, both at the process temperature of the heat-treatment process and at a temperature of a subsequent further treatment. During each of these further treatments, the temperature is always below the process temperature of the heat-treatment process. Tungsten (W), tantalum (Ta) or zirconium (Zr) are suitable metals, for example, in this connection. The covering layer preferably has a thickness from 50 to 250 nm. In this thickness range, the covering layer is sufficiently impervious to the aggressive substances.
Advantageous embodiments of the method, which emerge from the corresponding subclaims, have essentially the same advantages as the abovementioned respectively corresponding embodiments of the semiconductor device itself.
Other refinements of the method relate to the application of the material to the n-conducting semiconductor region and to the heat-treatment process.
In one refinement of the method, the material which is applied to the semiconductor region is taken from two separate sources. In this case, the sources each contain one of the two material components. They are taken by simultaneous vaporization or sputtering. The contact layer is formed by depositing the two material components on the n-conductor semiconductor region. In this case, the material of the contact layer is produced either while still in the vapor phase from the two material components, in the course of the deposition process, or not until afterward. Appropriately set process parameters can ensure that a specific intended mixing ratio of the two material components is adhered to.
In this case, the sputtering from two separate sources, can be effected in such a way that, alternately in a short time sequence, in each case only one of the two material components from the associated source is sputtered and deposited as a monofilm on the n-conducting semiconductor region. The resulting monofilms are very thin. They have, in particular, just a thickness of the order of magnitude of a few xc3x85ngstrxc3x6ms. In the extreme case, such a monofilm may also consist just of a single atomic layer, a so-called monolayer. On account of the small layer thickness and the short time sequence in the course of the layer deposition, this material application of the two material components is also designated as simultaneous here. Mixing together of the atoms of these monolayers (homogenization) then takes place, depending on the process conditions, at least in part already during the application process itself or right at the beginning of the subsequent heat-treatment process. On account of the small layer thickness, this mixing-together process lasts only a very short time.
By contrast, an alternative refinement provides for first preparing a source material (alloy target) from the first and second material components to be then sputtered in a second method step. The released particles of the source material are then deposited as contact layer on the n-conducting SiC semiconductor region, as in the previously described refinement. Vaporization of the source material from the alloy source is likewise possible.
In an advantageous embodiment variant, the semiconductor device is heated during the heat-treatment process to a process temperature of at most 900xc2x0 C., in particular to about 850xc2x0 C. This process temperature is then preferably kept approximately constant for up to two hours, in particular for two minutes. Specifically, the heat-treatment process can, if required, also be carried out at a lower temperature, such as e.g. at 800xc2x0 C., but in return over a longer period of time, e.g. 30 minutes. A considerably longer heat-treatment time in the region of several hours is also possible. On the other hand, the heat-treatment process can, however, also consist only of a heating phase and an immediately following cooling phase, without a hold time at a process temperature being provided in between. The heating and cooling operation is preferably carried out using a so-called RTP (Rapid Thermal Processing) installation or using a so-called RIA (Rapid Isothermal Annealing) installation. The heat-treatment process serves for the ohmic forming of the contact layer. It has been found that a thermally stable contact with good ohmic characteristics and a low contact resistance results on the n-conducting SiC after this heat-treatment process.
A further refinement, in which the heat-treatment process takes place with the exclusion of oxygen, in particular in an inert gas atmosphere, is additionally favorable. A possible inert gas is argon (Ar) or helium (He), for example. However, nitrogen (N) or hydrogen (H) can also advantageously be used for producing the oxygen exclusion. Oxygen is undesirable owing to its high reactivity in particular with the iron of a nickel/iron alloy optionally used for the contact layer.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a semiconductor device with ohmic contact-connection and method for the ohmic contact-connection of a semiconductor device, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.